1. Field of the Invention
The present invention relates to information communication system testing. More particularly, the present invention relates to a system and method for identifying a signature of a device, in a communication circuit, utilizing distortion products.
2. Background Information
The presence of Local Exchange Carrier (LEC) telecom wire line devices deployed as Customer Premises Equipment (CPE) or Central Office (CO) equipment can be difficult to distinguish electrically from the characteristics of another loop which is totally unterminated, when such devices are installed on a typical twisted pair loop. Such devices can include, for example, CPE splitters for separating a Digital Subscriber Loop (DSL) service from a Plain Old Telephone Service (POTS) delivered on the same loop pair, CO splitters, Integrated Services Digital Network (ISDN) modems, Maintenance Termination Units (MTUs) and the like. These and other like devices can be fitted with an electrical signature that is elicited by stimulus outside of the range of encountered conditions associated with the type of service commissioned for the subject loop, so that their presence or absence can more readily be determined without physical examination. For example, signatures associated with CO devices can display asymmetric voltage verses current (EI) curves about the origin, appearing electrically absent under the polarity and range of normal POTS voltages. However, these devices can display an identifying characteristic curve slope and inflection point or points under the influence of reverse polarity voltages. This class of signatures can be identified by recording loop direct current (DC) in response to a selected variety of applied DC loop voltages.
In contrast, for the class of devices utilized as CPE, an Alternating Current (AC) signature EI curve is generally utilized that is symmetric about the origin, yet appearing electrically absent until some characteristic minimum, polarity independent, loop voltage is reached. At or beyond this polarity independent applied voltage, a characteristic additional impedance is impressed upon the subject loop. The symmetric operation is necessary as the certainty that the individual leads (the so-called Tip and Ring leads) comprising the subject telecom pair are labeled consistently from CO to CPE is poor. Under such conditions, an asymmetric signature could be missed if the pair leads were reversed, or the signature could be elicited by normal loop working conditions and consequently interfere with normal operation. Thus, since normal POTS loop working conditions can include a large DC voltage for the purposes of power and supervision (e.g., on and off hook, and dialing) for POTS telephones, and since the Tip and Ring lead orientation is uncertain at the CPE, any DC-dependent signature component is inappropriate.
Conventionally, to identify this class of signatures, a sinusoidal AC signal with peak values below the characteristic minimum turn on value is applied. The resultant AC impedance of the circuit pair is then measured without the influence of the signature. Next, a sinusoidal AC signal with peak values substantially in excess (e.g., three or more times) of the turn-on threshold voltage is applied, so that the signature impedance appears in combination with the normal circuit pair impedance. The expected change in measured total impedance can be calculated and used as a criterion for decision of presence or absence. Unfortunately, for long loops, and especially long loops that have certain CPE connected that possess significant capacitance, the change in observed impedance in response to a large AC stimulus may be sufficiently small so as to fall within the accuracy window of the measuring instrument, thus making the judgment of signature presence or absence uncertain.